The Prion Diseases

Prions, once dismissed as an impossibility, have now gained
wide recognition as extraordinary agents that cause a number of
infectious, genetic and spontaneous disorders

by Stanley B. Prusiner.

Description & History

Fifteen years ago I evoked a good deal of skepticism when I
proposed that the infectious agents causing certain degenerative
disorders of the central nervous system in animals and, more
rarely, in humans might consist of protein and nothing else. At
the time, the notion was heretical. Dogma held that the conveyers
of transmissible diseases required genetic material, composed of
nucleic acid (DNA or RNA), in order to establish an infection in
a host. Even viruses, among the simplest microbes, rely on such
material to direct synthesis of the proteins needed for survival
and replication.

Later, many scientists were similarly dubious
when my colleagues and I suggested that these "proteinaceous
infectious particles"--or "prions," as I called the
disease-causing agents--could underlie inherited, as well as
communicable, diseases. Such dual behavior was then unknown to
medical science.

And we met resistance again when we concluded
that prions (pronounced "pree-ons") multiply in an incredible
way; they convert normal protein molecules into dangerous ones
simply by inducing the benign molecules to change their shape.
Today, however, a wealth of experimental and clinical data has
made a convincing case that we are correct on all three counts.

Prions are indeed responsible for transmissible and inherited
disorders of protein conformation. They can also cause sporadic
disease, in which neither transmission between individuals nor
inheritance is evident. Moreover, there are hints that the prions
causing the diseases explored thus far may not be the only ones.
Prions made of rather different proteins may contribute to other
neurodegenerative diseases that are quite prevalent in humans.
They might even participate in illnesses that attack muscles.

The known prion diseases, all fatal, are sometimes referred to as
spongiform encephalopathies. They are so named because they
frequently cause the brain to become riddled with holes. These
ills, which can brew for years (or even for decades in humans)
are widespread in animals. The most common form is scrapie,
found in sheep and goats. Afflicted animals lose coordination and
eventually become so incapacitated that they cannot stand. They
also become irritable and, in some cases, develop an intense itch
that leads them to scrape off their wool or hair (hence the name
"scrapie"). The other prion diseases of animals go by such names
as transmissible mink encephalopathy, chronic wasting disease of
mule deer and elk, feline spongiform encephalopathy and bovine
spongiform encephalopathy. The last, often called mad cow
disease, is the most worrisome.

Gerald A. H. Wells and John W.
Wilesmith of the Central Veterinary Laboratory in Weybridge,
England, identified the condition in 1986, after it began
striking cows in Great Britain, causing them to became
uncoordinated and unusually apprehensive. The source of the
emerging epidemic was soon traced to a food supplement that
included meat and bone meal from dead sheep. The methods for
processing sheep carcasses had been changed in the late 1970s.
Where once they would have eliminated the scrapie agent in the
supplement, now they apparently did not. The British government
banned the use of animal-derived feed supplements in 1988, and
the epidemic has probably peaked. Nevertheless, many people
continue to worry that they will eventually fall ill as a result
of having consumed tainted meat.

The human prion diseases are more obscure. Kuru has been seen
only among the Fore Highlanders of Papua New Guinea. They call it
the "laughing death." Vincent Zigas of the Australian Public
Health Service and D. Carleton Gajdusek of the U.S. National
Institutes of Health described it in 1957, noting that many
highlanders became afflicted with a strange, fatal disease marked
by loss of coordination (ataxia) and often later by dementia. The
affected individuals probably acquired kuru through ritual
cannibalism: the Fore tribe reportedly honored the dead by eating
their brains. The practice has since stopped, and kuru has
virtually disappeared.

Creutzfeldt-Jakob disease, in contrast,
occurs worldwide and usually becomes evident as dementia. Most of
the time it appears sporadically, striking one person in a
million, typically around age 60. About 10 to 15 percent of cases
are inherited, and a small number are, sadly, iatrogenic--spread
inadvertently by the attempt to treat some other medical problem.
Iatrogenic Creutzfeldt-Jakob disease has apparently been
transmitted by corneal transplantation, implantation of dura
mater or electrodes in the brain, use of contaminated surgical
instruments, and injection of growth hormone derived from human
pituitaries (before recombinant growth hormone became available).

The two remaining human disorders are
Gerstmann-Straussler-Scheinker disease (which is (2)
manifest as ataxia and other signs of damage to the cerebellum)
and fatal familial insomnia (in which dementia follows difficulty
sleeping).
Both these conditions are usually inherited and typically appear
in midlife. Fatal familial insomnia was discovered only recently,
by Elio Lugaresi and Rossella Medori of the University of Bologna
and Pierluigi Gambetti of Case Western Reserve University.

In Search of the Cause

I first became intrigued by the prion diseases in 1972,
when as a resident in neurology at the University of California
School of Medicine at San Francisco, I lost a patient to
Creutzfeldt-Jakob disease. As I reviewed the scientific
literature on that and related conditions, I learned that
scrapie, Creutzfeldt-Jakob disease and kuru had all been shown to
be transmissible by injecting extracts of diseased brains into
the brains of healthy animals. The infections were thought to be
caused by a slow-acting virus, yet no one had managed to isolate
the culprit.

In the course of reading, I came across an
astonishing report in which Tikvah Alper and her colleagues at
the Hammersmith Hospital in London suggested that the scrapie
agent might lack nucleic acid, which usually can be degraded by
ultraviolet or ionizing radiation. When the nucleic acid in
extracts of scrapie-infected brains was presumably destroyed by
those treatments, the extracts retained their ability to transmit
scrapie. If the organism did lack DNA and RNA, the finding would
mean that it was not a virus or any other known type of
infectious agent, all of which contain genetic material. What,
then, was it?

Investigators had many ideas--including, jokingly,
linoleum and kryptonite--but no hard answers. I immediately
began trying to solve this mystery when I set up a laboratory at
U.C.S.F. in 1974. The first step had to be a mechanical
one--purifying the infectious material in scrapie-infected brains
so that its composition could be analyzed. The task was daunting;
many investigators had tried and failed in the past. But with the
optimism of youth, I forged ahead [see "Prions," by Stanley B.
Prusiner; SCIENTIFIC AMERICAN, October 1984].

Amazing Discovery

All our results pointed toward one startling conclusion:
the infectious agent in scrapie (and presumably in the related
diseases) did indeed lack nucleic acid and consisted mainly, if
not exclusively, of protein. We deduced that DNA and RNA were
absent because, like Alper, we saw that procedures known to
damage nucleic acid did not reduce infectivity. And we knew
protein was an essential component because procedures that
denature (unfold) or degrade protein reduced infectivity.

I thus
introduced the term "prion" to distinguish this class of disease
conveyer from viruses, bacteria, fungi and other known pathogens.
Not long afterward, we determined that scrapie prions contained a
single protein that we called PrP, for "prion protein." Now the
major question became; Where did the instructions specifying the
sequence of amino acids in PrP reside? Were they carried by an
undetected piece of DNA that traveled with PrP, or were they,
perhaps, contained in a gene housed in the chromosomes of cells?

The key to this riddle was the identification in 1984 of some 15
amino acids at one end of the PrP protein. My group identified
this short amino acid sequence in collaboration with Leroy E.
Hood and his co-workers at the California Institute of
Technology. Knowledge of the sequence allowed us and others to
construct molecular probes, or detectors, able to indicate
whether mammalian cells carried the PrP gene. With probes
produced by Hood's team, Bruno Oesch, working in the laboratory
of Charles Weissmann at the University of Zurich, showed that
hamster cells do contain a gene for PrP. At about the same time,
Bruce Cheseboro of the NIH Rocky Mountain Laboratories made his
own probes and established that mouse cells harbor the gene as
well. That work made it possible to isolate the gene and to
establish that it resides not in prions but in the chromosomes of
hamsters, mice, humans and all other mammals that have been
examined. What is more, most of the time, these animals make PrP
without getting sick.

One interpretation of such findings was
that we had made a terrible mistake: PrP had nothing to do with
prion diseases. Another possibility was that PrP could be
produced in two forms, one that generated disease and one that
did not. We soon showed the latter interpretation to be
correct. The critical clue was the fact that the PrP found in
infected brains resisted breakdown by cellular enzymes called
proteases.

Most proteins in cells are degraded fairly easily. I therefore
suspected that if a normal, (3) nonthreatening form
of PrP existed, it too would be susceptible to degradation.
Ronald A. Barry in my laboratory then identified this
hypothetical protease-sensitive form. It thus became clear that
scrapie-causing PrP is a variant of a normal protein. We
therefore called the normal protein "cellular PrP" and the
infectious (protease-resistant) form "scrapie PrP." The latter
term is now used to refer to the protein molecules that
constitute the prions causing all scrapie-like diseases of
animals and humans.

Prion Diseases Can Be Inherited

Early on we had hoped to use the PrP gene to generate pure
copies of PrP. Next, we would inject the protein molecules into
animals, secure in the knowledge that no elusive virus was
clinging to them. If the injections caused scrapie in the
animals, we would have shown that protein molecules could, as we
had proposed, transmit disease. By 1986, however, we knew the
plan would not work. For one thing, it proved very difficult to
induce the gene to make the high levels of PrP needed for
conducting studies. For another thing, the protein that was
produced was the normal, cellular form. Fortunately, work on a
different problem led us to an alternative approach for
demonstrating that prions could transmit scrapie without the help
of any accompanying nucleic acid.

In many cases, the
scrapielike illnesses of humans seemed to occur without having
been spread from one host to another, and in some families they
appeared to be inherited. (Today researchers know that about 10
percent of human prion diseases are familial, felling half of the
members of the affected families.) It was this last pattern that
drew our attention. Could it be that prions were more unusual
than we originally thought? Were they responsible for the
appearance of both hereditary and transmissible illnesses? In
1988 Karen Hsiao in my laboratory and I uncovered some of the
earliest data showing that human prion diseases can certainly be
inherited. We acquired clones of a PrP gene obtained from a man
who had Gerstmann-Straussler-Scheinker disease in his family and
was dying of it himself. Then we compared his gene with PrP genes
obtained from a healthy population and found a tiny abnormality
known as a point mutation.

To grasp the nature of this
mutation, it helps to know something about the organization of
genes. Genes consist of two strands of the DNA building blocks
called nucleotides, which differ from one another in the bases
they carry. The bases on one strand combine with the bases on the
other strand to form base pairs: the "rungs" on the familiar DNA
"ladder." In addition to holding the DNA ladder together, these
pairs spell out the sequence of amino acids that must be strung
together to make a particular protein. Three base pairs
together--a unit called a codon--specify a single amino acid. In
our dying patient, just one base pair (out of more than 750) had
been exchanged for a different pair. The change, in turn, had
altered the information carried by codon 102, causing the amino
acid leucine to be substituted for the amino acid proline in the
man's PrP protein.

With the help of Tim J. Crow of Northwick
Park Hospital in London and Jurg Ott of Columbia University and
their colleagues, we discovered the same mutation in genes from a
large number of patients with Gerstmann-Straussler-Scheinker
disease, and we showed that the high incidence in the affected
families was statistically significant. In other words, we
established genetic linkage between the mutation and the
disease--a finding that strongly implies the mutation is the
cause. Over the past six years work by many investigators has
uncovered 18 mutations in families with inherited prion diseases;
for five of these mutations, enough cases have now been collected
to demonstrate genetic linkage. The discovery of mutations gave
us a way to eliminate the possibility that a nucleic acid was
traveling with prion proteins and directing their multiplication.

We could now create genetically altered mice carrying a mutated
PrP gene. If the presence of the altered gene in these
"transgenic" animals led by itself to scrapie, and if the brain
tissue of the transgenic animals then caused scrapie in healthy
animals, we would have solid evidence that the protein encoded by
the mutated gene had been solely responsible for the transfer of
disease. Studies I conducted with Hsiao, Darlene Groth in my
group and Stephen J. DeArmond, head of a separate laboratory at
U.C.S.F., have now shown that scrapie can be generated and
transmitted in this way [see BOX at end of this article]. These
results in animals resemble those obtained in 1981, when
Gajdusek, Colin L. Masters and Clarence J. Gibbs, Jr., all at the
National Institutes of Health, transmitted apparently inherited
Gerstmann-Straussler-Scheinker disease to monkeys. They also
resemble the findings of Jun Tateishi and Tetsuyuki Kitamoto of
Kyushu University in Japan, who transmitted inherited
Creutzfeldt-Jakob disease to mice.

Together the collected transmission studies persuasively argue
that prions do, after all, (4) represent an
unprecedented class of infectious agents, composed
only of a modified mammalian protein. And the conclusion is
strengthened by the fact that assiduous searching for a
scrapie-specific nucleic acid (especially by Detlev H. Riesner of
Heinrich Heine University in Dusseldorf) has produced no evidence
that such genetic material is attached to prions. Scientists who
continue to favor the virus theory might say that we still have
not proved our case. If the PrP gene coded for a protein that,
when mutated, facilitated infection by a ubiquitous virus, the
mutation would lead to viral infection of the brain.
Then injection of brain extracts from the mutant animal would
spread the infection to another host. Yet in the absence of any
evidence of a virus, this hypothesis looks to be untenable.

In
addition to showing that a protein can multiply and cause disease
without help from nucleic acids, we have gained insight into how
scrapie PrP propagates in cells. Many details remain to be worked
out, but one aspect appears quite clear: the main difference
between normal PrP and scrapie PrP is conformational. Evidently,
the scrapie protein propagates itself by contacting normal PrP
molecules and somehow causing them to unfold and flip from their
usual conformation to the scrapie shape. This change initiates a
cascade in which newly converted molecules change the shape of
other normal PrP molecules, and so on. These events apparently
occur on a membrane in the cell interior.

We started to think
that the differences between cellular and scrapie forms of PrP
must be conformational after other possibilities began to seem
unlikely. For instance, it has long been known that the
infectious form often has the same amino acid sequence as the
normal type. Of course, molecules that start off being identical
can later be chemically modified in ways that alter their
activity. But intensive investigations by Neil Stahl and Michael
A. Baldwin in my laboratory have turned up no differences of this
kind.

One Protein, Two Shapes

How, exactly, do the structures of normal and scrapie forms
of PrP differ? Studies by Keh-Ming Pan in our group indicate that
the normal protein consists primarily of alpha helices, regions
in which the protein backbone twists into a specific kind of
spiral; the scrapie form, however, contains beta strands, regions
in which the backbone is fully extended. Collections of these
strands form beta sheets. Fred E. Cohen, who directs another
laboratory at U.C.S.F., has used molecular modeling to try to
predict the structure of the normal protein based on its amino
acid sequence. His calculations imply that the protein probably
folds into a compact structure having four helices in its core.

Less is known about the structure, or structures, adopted by
scrapie PrP. The evidence supporting the proposition that
scrapie PrP can induce an alpha-helical PrP molecule to switch to
a beta-sheet form comes primarily from two important studies by
investigators in my group. Maria Gasset learned that synthetic
peptides (short strings of amino acids) corresponding to three of
the four putative alpha-helical regions of PrP can fold into beta
sheets. And Jack Nguyen has shown that in their beta-sheet
conformation, such peptides can impose a beta-sheet structure on
helical PrP peptides. More recently Byron W. Caughey of the Rocky
Mountain Laboratories and Peter T. Lansbury of the Massachusetts
Institute of Technology have reported that cellular PrP can be
converted into scrapie PrP in a test tube by mixing the two
proteins together.

PrP molecules arising from mutated genes
probably do not adopt the scrapie conformation as soon as they
are synthesized. Otherwise, people carrying mutant genes would
become sick in early childhood. We suspect that mutations in the
PrP gene render the resulting proteins susceptible to flipping
from an alpha-helical to a beta-sheet shape. Presumably, it takes
time until one of the molecules spontaneously flips over and
still more time for scrapie PrP to accumulate and damage the
brain enough to cause symptoms. Fred Cohen and I think we might
be able to explain why the various mutations that have been noted
in PrP genes could facilitate folding into the beta-sheet form.

Many of the human mutations give rise to the substitution of one
amino acid for another within the four putative helices or at
their borders. Insertion of incorrect amino acids at those
positions might destabilize a helix, thus increasing the
likelihood that the affected helix and its neighbors will refold
into a beta-sheet conformation. Conversely, Hermann Schatzel in
my laboratory finds that the harmless differences distinguishing
the PrP gene of humans from those of apes and monkeys affect
amino acids lying outside of the proposed helical domains--where
the divergent amino acids probably would not profoundly influence
the stability of the helical regions.

Treatment Ideas Emerge(5)

No one knows exactly how propagation of scrapie PrP damages
cells. In cell cultures, the conversion of normal PrP to the
scrapie form occurs inside neurons, after which scrapie PrP
accumulates in intracellular vesicles known as lysosomes. In the
brain, filled lysosomes could conceivably burst and damage cells.
As the diseased cells died, creating holes in the brain, their
prions would be released to attack other cells. We do know with
certainty that cleavage of scrapie PrP is what produces PrP
fragments that accumulate as plaques in the brains of some
patients. Those aggregates resemble plaques seen in Alzheimer's
disease, although the Alzheimer's clumps consist of a different
protein. The PrP plaques are a useful sign of prion infection,
but they seem not to be a major cause of impairment. In many
people and animals with prion disease, the plaques do not arise
at all.

Even though we do not yet know much about how PrP scrapie
harms brain tissue, we can foresee that an understanding of the
three-dimensional structure of the PrP protein will lead to
therapies. If, for example, the four-helix-bundle model of PrP is
correct, drug developers might be able to design a compound that
would bind to a central pocket that could be formed by the four
helices. So bound, the drug would stabilize these helices and
prevent their conversion into beta sheets.

Another idea for
therapy is inspired by research in which Weissmann and his
colleagues applied gene-targeting technology to create mice that
lacked the PrP gene and so could not make PrP. By knocking out a
gene and noting the consequences of its loss, one can often
deduce the usual functions of the gene's protein product. In this
case, however, the animals missing PrP displayed no detectable
abnormalities. If it turns out that PrP is truly inessential,
then physicians might one day consider delivering so-called
antisense or antigene therapies to the brains of patients with
prion diseases. Such therapies aim to block genes from giving
rise to unwanted proteins and could potentially shut down
production of cellular PrP [see "The New Genetic Medicines," by
Jack S. Cohen and Michael E. Hogan; SCIENTIFIC AMERICAN, December
1994].

They would thereby block PrP from propagating itself. It
is worth noting that the knockout mice provided a welcomed
opportunity to challenge the prion hypothesis. If the animals
became ill after inoculation with prions, their sickness would
have indicated that prions could multiply even in the absence of
a preexisting pool of PrP molecules. As I expected, inoculation
with prions did not produce scrapie, and no evidence of prion
replication could be detected.

The enigma of how scrapie PrP
multiplies and causes disease is not the only puzzle starting to
be solved. Another long-standing question--the mystery of how
prions consisting of a single kind of protein can vary markedly
2n their effects--is beginning to be answered as well. Lain H.
Pattison of the Agriculture Research Council in Compton, England,
initially called attention to this phenomenon. Years ago he
obtained prions from two separate sets of goats. One isolate made
inoculated animals drowsy, whereas the second made them
hyperactive. Similarly, it is now evident that some prions cause
disease quickly, whereas others do so slowly.

The Mystery of "Strains"

Alan G. Dickinson, Hugh Fraser and Moira E. Bruce of the
Institute for Animal Health in Edinburgh, who have examined the
differential effects of varied isolates in mice, are among those
who note that only pathogens containing nucleic acids are known
to occur in multiple strains. Hence, they and others assert, the
existence of prion "strains" indicates the prion hypothesis must
be incorrect; viruses must be at the root of scrapie and its
relatives. Yet because efforts to find viral nucleic acids have
been unrewarding, the explanation for the differences must lie
elsewhere. One possibility is that prions can adopt multiple
conformations. Folded in one way, a prion might convert normal
PrP to the scrapie form highly efficiently, giving rise to short
incubation times. Folded another way, it might work less
efficiently. Similarly, one "conformer" might be attracted to
neuronal populations in one part of the brain, whereas another
might be attracted to neurons elsewhere, thus producing different
symptoms. Considering that PrP can fold in at least two ways, it
would not be surprising to find it can collapse into other
structures as well.

Since the mid-1980s we have also sought
insight into a phenomenon known as the species barrier. This
concept refers to the fact that something makes it difficult for
prions made by one species to cause disease in animals of another
species. The cause of this difficulty is of considerable interest
today because of the epidemic of mad cow disease in Britain.
We and others have been trying to find out whether the species
barrier is strong (6)
enough to prevent the spread of prion disease from cows to
humans.

Breaking the Barrier

The barrier was discovered by Pattison, who in the 1960s
found it hard to transmit scrapie between sheep and rodents. To
determine the cause of the trouble, my colleague Michael R. Scott
and I later generated transgenic mice expressing the PrP gene of
the Syrian hamster--that is, making the hamster PrP protein. The
mouse gene differs from that of the hamster gene at 16 codons out
of 254. Normal mice inoculated with hamster prions rarely acquire
scrapie, but the transgenic mice became ill within about two
months. We thus concluded that we had broken the species barrier
by inserting the hamster genes into the mice. Moreover, on the
basis of this and other experiments, we realized that the barrier
resides in the amino acid sequence of PrP: the more the sequence
of a scrapie PrP molecule resembles the PrP sequence of its host,
the more likely it is that the host will acquire prion disease.

In one of those other experiments, for example, we examined
transgenic mice carrying the Syrian hamster PrP gene in addition
to their own mouse gene. Those mice make normal forms of both
hamster and mouse PrP. When we inoculated the animals with mouse
prions, they made more mouse prions. When we inoculated them with
hamster prions, they made hamster prions.
From this behavior, we learned that prions preferentially
interact with cellular PrP of homologous, or like, composition.
The attraction of scrapie PrP for cellular PrP having the same
sequence probably explains why scrapie managed to spread to cows
in England from food consisting of sheep tissue: sheep and bovine
PrP differ only at seven positions. In contrast, the sequence
difference between human and bovine PrP is large: the molecules
diverge at more than 30 positions. Because the variance is great,
the likelihood of transmission from cows to people would seem to
be low.

Consistent with this assessment are epidemiological
studies by W. Bryan Matthews, a professor emeritus at the
University of Oxford. Matthews found no link between scrapie in
sheep and the occurrence of Creutzfeldt-Jakob disease in
sheep-farming countries. On the other hand, two farmers who had
"mad cows" in their herds have recently died of Creutzfeldt-Jakob
disease. Their deaths may have nothing to do with the bovine
epidemic, but the situation bears watching. It may turn out that
certain parts of the PrP molecule are more important than others
for breaking the species barrier.

If that is the case, and if cow
PrP closely resembles human PrP in the critical regions, then the
likelihood of danger might turn out to be higher than a simple
comparison of the complete amino acid sequences would suggest.
We began to consider the possibility that some parts of the PrP
molecule might be particularly important to the species barrier
after a study related to this blockade took an odd turn. My
colleague Glenn C. Telling had created transgenic mice carrying a
hybrid PrP gene that consisted of human codes flanked on either
side by mouse codes; this gene gave rise to a hybrid protein.
Then he introduced brain tissue from patients who had died of
Creutzfeldt-Jakob disease or Gerstmann-Straussler-Scheinker
disease into the transgenic animals.

Oddly enough, the animals
became ill much more frequently and faster than did mice carrying
a full human PrP gene, which diverges from mouse PrP at 28
positions. This outcome implied that similarity in the central
region of the PrP molecule may be more critical than it is in the
other segments. The result also lent support to earlier
indications--uncovered by Shu-Lian Yang in DeArmond's laboratory
and Albert Taraboulos in my group--that molecules made by the
host can influence the behavior of scrapie PrP. We speculate that
in the hybrid-gene study, a mouse protein, possibly a "chaperone"
normally involved in folding nascent protein chains, recognized
one of the two mouse-derived regions of the hybrid PrP protein.
This chaperone bound to that region and helped to refold the
hybrid molecule into the scrapie conformation. The chaperone did
not provide similar help in mice making a totally human PrP
protein, presumably because the human protein lacked a binding
site for the mouse factor.

The List May Grow

An unforeseen story has recently emerged from studies of
transgenic mice making unusually high amounts of normal PrP
proteins. DeArmond, David Westaway in our group and George A.
Carlson of the McLaughlin Laboratory in Great Falls, Mont.,
became perplexed when they noted that some older transgenic mice
developed an illness characterized by rigidity and diminished
grooming.

When we pursued the cause, we found that making excessive amounts
of PrP can (7)
eventually lead to neurodegeneration and, surprisingly, to
destruction of both muscles and peripheral nerves. These
discoveries widen the spectrum of prion diseases and are
prompting a search for human prion diseases that affect the
peripheral nervous system and muscles. Investigations of
animals that overproduce PrP have yielded another benefit as
well.

They offer a clue as to how the sporadic form of
Creutzfeldt-Jakob disease might arise. For a time I suspected
that sporadic disease might begin when the wear and tear of
living led to a mutation of the PrP gene in at least one cell in
the body. Eventually, the mutated protein might switch to the
scrapie form and gradually propagate itself, until the buildup of
scrapie PrP crossed the threshold to overt disease. The mouse
studies suggest that at some point in the lives of the one in a
million individuals who acquire sporadic Creutzfeldt-Jakob
disease, cellular PrP may spontaneously convert to the scrapie
form.

The experiments also raise the possibility that people who
become afflicted with sporadic Creutzfeldt-Jakob disease
overproduce PrP, but we do not yet know if, in fact, they do.
All the known prion diseases in humans have now been modeled in
mice. With our most recent work we have inadvertently developed
an animal model for sporadic prion disease. Mice inoculated with
brain extracts from scrapie-infected animals and from humans
afflicted with Creutzfeldt-Jakob disease have long provided a
model for the infectious forms of prion disorders. And the
inherited prion diseases have been modeled in transgenic mice
carrying mutant PrP genes.

These murine representations of the human prion afflictions
should not only extend understanding of how prions cause brain
degeneration, they should also create opportunities to evaluate
therapies for these devastating maladies.

Striking Similarities

Ongoing research may also help determine whether prions
consisting of other proteins play a part in more common
neurodegenerative conditions, including Alzheimer's disease,
Parkinson's disease and amyotrophic lateral sclerosis. There are
some marked similarities in all these disorders. As is true of
the known prion diseases, the more widespread ills mostly occur
sporadically but sometimes "run" in families. All are also
usually diseases of middle to later life and are marked by
similar pathology: neurons degenerate, protein deposits can
accumulate as plaques, and glial cells (which support and nourish
nerve cells) grow larger in reaction to damage to neurons.

Strikingly, in none of these disorders do white blood
cells--those ever present warriors of the immune
system--infiltrate the brain. If a virus were involved in these
illnesses, white cells would be expected to appear. Recent
findings in yeast encourage speculation that prions unrelated in
amino acid sequence to the PrP protein could exist. Reed B.
Wickner of the NIH reports that a protein called Ure2p might
sometimes change its conformation, thereby affecting its activity
in the cell. In one shape, the protein is active; in the other,
it is silent. The collected studies described here argue
persuasively that the prion is an entirely new class of
infectious pathogen and that prion diseases result from
aberrations of protein conformation. Whether changes in protein
shape are responsible for common neurodegenerative diseases, such
as Alzheimer's, remains unknown, but it is a possibility that
should not be ignored.

BOX: A Persuasive Experiment

Several studies have shown that prions composed only of PrP
are able to convey infection from one animal to another. In one
such experiment, the author and his colleagues created mice
carrying many copies of a mutant PrP gene; these animals made
high levels of mutant PrP, some of which appears to adopt the
scrapie conformation. Eventually all the mice displayed symptoms
of brain damage and died.

Then the workers injected brain tissue
from the diseased animals into genetically altered mice making
low levels of the same mutant PrP protein. (Such mice were chosen
as recipients because scrapie PrP is most attracted to PrP
molecules having the same composition.) Uninoculated mice did not
become ill (indicating that making low levels of the aberrant
protein was safe), but many of the treated ones did. Moreover,
brain tissue transferred from the diseased recipients to their
healthy counterparts caused illness once again. If the aberrant
protein were unable to transmit infection, none of the inoculated
animals would have sickened.

STANLEY B. PRUSINER is professor of neurology and
biochemistry at the University of (8) California School of
Medicine, San Francisco. He is a member of the National Academy
of Sciences, the Institute of Medicine and the American Academy
of Arts and Sciences. He has won many awards for his research
into prions, most recently the Albert Lasker Basic Medical
Research Award and the Paul Ehrlich Award. This is his second
article for Scientific American.